(Reuters) — Underground sensors have
detected excessive radiation levels inside a nuclear waste storage site
deep below New Mexico's desert, but no workers have been exposed and
there was no risk to public health, U.S. Department of Energy officials
said on Sunday.

An air-monitoring alarm went off at 11:30 p.m. local time Friday
indicating unsafe concentrations of radiation inside the Waste
Isolation Pilot Plant in what DOE officials said appeared to be the
first such mishap since the facility opened in 1999.

As of Sunday, the source of the high radiation readings had yet to
be determined, and a plan to send inspection teams below ground to
investigate was put on hold as a precaution.

"They will not go in today. It's a safety thing more than anything.
We're waiting until we get other assessments done before we
authorize re-entry," DOE spokesman Bill Mackie said.

The facility, located in southeastern New Mexico near Carlsbad, is
designed as a repository for so-called transuranic waste, which
includes discarded machinery, clothing and other materials
contaminated with plutonium or other radioisotopes heavier than
uranium.

The waste, shipped in from other DOE nuclear laboratories and
weapons sites around the country, is buried in underground salt
formations that gradually close in around the disposal casks and
seal them from the outside world.

No workers were underground when the apparent radiation leak was
detected in the vicinity of the plant's waste-disposal platform, and
none of the 139 employees working above ground at the time was
exposed, the Energy Department said.

The alarm automatically switched the underground ventilation system
to filtration to keep any releases from reaching the surface, DOE
officials said.

Subsequent testing of surface air in and around the facility showed
the incident posed no danger to human health or the environment,
Mackie said.

Air-monitor alarms at the facility have been tripped in the past by
malfunctions or fluctuations in levels of radon, a naturally
occurring radioactive gas. But officials said they believe this to
be the first real alarm since the plant began operations.

Just a few dozen essential personnel, including security officers,
remained at the site over the weekend.

Inbound waste shipments had already been suspended at the plant
since a truck caught fire there earlier this month in an accident
that left several workers suffering smoke inhalation.

"We're in shutdown mode," Mackie said.

The facility in the Chihuahuan Desert normally receives up to 6,000
cubic meters of radioactive waste a year and employs more than 800
workers. The site is expected to continue to accept radiological
materials until 2030, Mackie said.